1. Field of Invention
The invention relates to a frequency controller for adjusting power conversion and, in particular, to a adaptive dual-slope frequency controller for adjusting power conversion.
2. Related Art
In the circuit of the power supply 2 shown in FIG. 1, the power converter 4 is used to receive an external input voltage Vin and to provide an output voltage Vo to the load device 6. The feedback control circuit 8 outputs an appropriate gate pulse to the power converter 4 according to the output voltage Vo in order to provide an appropriate power to the load device 6. For example, when the load device 6 is at heavy load, the feedback control circuit 8 makes the power converter 4 to provide a larger power output to satisfy the system's needs. On the other hand, if the load device 6 is at light load, the feedback control circuit 8 makes the power converter 4 to provide a smaller power output to save energy. The power converter 4 can be a buck converter, a boost converter, a fly back converter, or a forward converter, depending upon different specification needs. The feedback control circuit 8 mostly adjusts the output power of the power converter 4 by pulse width modulation (PWM). From the above description, it is seen that the feedback control circuit 8 is the key role of affecting the efficiency of a power supply 2.
FIG. 2 is the block diagram of a conventional feedback control circuit 8. It includes an error amplifier 11, a reference voltage generator 12, a comparator, 13, an oscillator 14, an SR inverter 15, a gate drive 16, and feedback compensation circuits 17, 18.
We use FIG. 3 to describe the principle of the conventional feedback control circuit 8. When an output voltage Vo enters the positive terminal of the error amplifier 11 via the feedback compensation circuit 18, the error amplifier 11 compares it with a reference voltage Vref generated by the reference voltage generator 12. It further feeds an amplified error voltage Ve to the positive terminals of the feedback compensation circuit 17 and the comparator 13. The feedback compensation circuits 17, 18 are circuits composed of resistors and capacitors. Their purpose is to stabilize the closed-loop feedback compensation of the power supply 2.
The comparator 13 compares the error voltage Ve and the voltage on the switch current CS of the switch chip (not shown) in order to generate a reset signal to the SR flip-flop 15. If the voltage is smaller than the voltage on the switch current CS of the switch chip, then the reset signal is a low voltage and so is the gate pulse. If the voltage is greater than the voltage on the switch current CS of the switch chip, then the reset signal is a high voltage and the voltage of the gate pulse is determined by the oscillation output signal CLKOUT of the oscillator 14. On the other hand, the oscillation output signal CLKOUT generated by the oscillator 14 periodically restores the gate pulse to the high voltage.
Therefore, the pulse width of the gate pulse determines the output power of the power converter 4. However, the frequency of the oscillation output signal CLKOUT is fixed. This in turn infers that the frequency of the gate pulse is also fixed. A fixed gate pulse will result in large power consumption in light load and no load conditions.